1. Field of the Invention
The present invention pertains to tantalum films having ultra-low resistivity, in the range of about 10 xcexcxcexa9-cm, as well as methods for depositing ultra-low resistivity tantalum films. Tantalum films deposited according to the method of the invention can be removed from a semiconductor substrate surface using chemical mechanical polishing (CMP) techniques far more rapidly than previously known tantalum films.
2. Brief Description of the Background Art
As microelectronics continue to miniaturize, interconnection performance, reliability, and power consumption has become increasingly important, and interest has grown in replacing aluminum alloys with lower resistivity and higher reliability metals. Copper offers a significant improvement over aluminum as a contact and interconnect material. For example, the resistivity of copper is about 1.67 xcexcxcexa9-cm, which is only about half of the resistivity of aluminum.
One of the preferred technologies which enables the use of copper interconnects is the damascene process. This process for producing a multi-level structure having feature sizes in the range of 0.5 micron (xcexcm) or less typically includes the following steps: blanket deposition of a dielectric material over a substrate; patterning of the dielectric material to form openings; deposition of a diffusion barrier layer and, optionally, a wetting layer to line the openings; deposition of a copper layer onto the substrate in sufficient thickness to fill the openings; and removal of excessive conductive material from the substrate surface using chemical-mechanical polishing (CMP) techniques. The damascene process is described in detail by C. Steinbruchel in xe2x80x9cPatterning of copper for multilevel metallization: reactive ion etching and chemical-mechanical polishingxe2x80x9d, Applied Surface Science 91 (1995) 139-146.
The preferred barrier layer/wetting layer for use with copper comprises a tantalum nitridexe2x80x94tantalum barrier/wetting layer having a decreasing nitrogen content toward the upper surface of the layer. This structure, which provides excellent barrier properties while increasing the  less than 111 greater than  content of an overlying copper layer, provides a copper layer having improved electromigration resistance, as described in applicants"" copending application Ser. No. 08/995,108. A barrier layer having a surface which is essentially pure tantalum or tantalum including only a small amount of nitrogen (typically less than about 15 atomic percent) performs well as a barrier layer and also as a wetting layer to enhance the subsequent application of an overlying copper layer.
Philip Catania et al. in xe2x80x9cLow resistivity body-centered cubic tantalum thin films as diffusion barriers between copper and siliconxe2x80x9d, J. Vac. Sci. Technol. A 10(5), September/October 1992, describes the resistivity of thin bcc-tantalum films and xcex2-tantalum films. The resistivity for bcc-tantalum (xcex1-tantalum) films is said to be on the order of 30 xcexcxcexa9-cm, while the resistivity of the xcex2-tantalum films ranges from about 160-180 xcexcxcexa9-cm. A comparison of the effectiveness of thin bcc-Ta and xcex2-Ta layers as diffusion barrier to copper penetration into silicon shows that the bcc-Ta which exhibits low resistivity also performs well as a barrier layer up to 650xc2x0 C.
Kyung-Hoon Min et al. in xe2x80x9cComparative study of tantalum and tantalum nitrides (Ta2N and TaN) as a diffusion barrier for Cu metallizationxe2x80x9d, J. Vac. Sci. Technol. B 14(5), September/October 1996, discuss tantalum and tantalum nitride films of about 50 nm thickness deposited by reactive sputtering onto a silicon substrate. The performance of these films as a diffusion barrier between copper and silicon is also discussed. The diffusion barrier layer performance is said to be enhanced as nitrogen concentration in the film is increased.
U.S. Pat. No.3,607,384 to Frank D. Banks, issued Sep. 21, 1971, describes thin film resistors which utilize layers of tantalum or tantalum nitride. FIG. 1 in the ""385 patent shows the resistivity for a particular tantalum nitride film as a function of the sputtering voltage and FIG. 2 shows the resistivity as a function of the nitrogen content of the film. The lowest resistivity obtained under any conditions was about 179 xcexcxcexa9-cm.
U.S. Pat. No.3,819,976 to Chilton et al., issued Jun. 25, 1974, describes a tantalum-aluminum alloy attenuator for traveling wave tubes. In the background art section of this patent, there is a reference to tantalum film undergoing a phase transition from beta-tantalum to body-centered-cubic (alpha) tantalum at about 700xc2x0 C.
U.S. Pat. No.3,878,079 to Alois Schauer, issued Apr. 15, 1975, describes and claims a method of producing thin tantalum films which are body-centered cubic lattices. The films are deposited upon a glass substrate, and FIG. 2 of the ""079 patent shows resistivity for tantalum nitride films as a function of nitrogen content. U.S. Pat. No. 4,000,055 to Kumagai et al., issued Dec. 28, 1976, discloses a method of depositing nitrogen-doped beta-tantalum thin films. FIG. 2 of the ""055 patent also shows the resistivity of the film as a function of the nitrogen content of the film.
U.S. Pat. No. 4,364,099 to Koyama et al., issued Dec. 14, 1982, discloses a tantalum film capacitor having an xcex1-tantalum as a lower electrode, a chemical conversion layer of xcex1-tantalum as a dielectric, and an upper electrode. This references also discusses a phase transition of the tantalum film depending on the nitrogen concentration of the film. When the nitrogen content ranges from about 6 to about 12 percent, the resistivity of the tantalum thin film is said to be advantageously low, although no particular resistivity data is provided.
U.S. Pat. No. 5,221,449 to Colgan et al., issued Jun. 22, 1993, describes a method of making alpha-tantalum thin films. In particular, a seed layer of Ta(N) is grown upon a substrate by reactive sputtering of tantalum in a nitrogen-containing environment. A thin film of xcex1-tantalum is then formed over the Ta(N) seed layer. In the Background Art section of the patent, reference is made to the xe2x80x9cHandbook of Thin Film Technologyxe2x80x9d, McGraw-Hill, page 18-12 (1970), where it is reported that if the substrate temperature exceeds 600xc2x0 C., alpha phase tantalum film is formed. Further reference is made to an article by G. Feinstein and R. D. Huttemann, xe2x80x9cFactors Controlling the Structure of Sputtered Tantalum Filmsxe2x80x9d, Thin Solid Films, Vol. 16, pages 129-145 (1973). The authors are said to divide substrates into three groups: Group I, containing substrates onto which only beta-tantalum can be formed (including glass, quartz, sapphire, and metals such as copper and nickel); Group II, containing substrates onto which only alpha (bcc) tantalum can be grown (including substrates coated with 5000 xc3x85 thick metal films such as gold, platinum, or tungsten); and Group III, containing substrates which normally produce alpha-tantalum, but which can be induced to yield beta-tantalum or mixtures of alpha and beta by suitable treatment of the surface (i.e., substrates coated with 5,000 xc3x85 of molybdenum, silicon nitride, or stoichiometric tantalum nitride, Ta2N).
As the feature size of semiconductor devices becomes ever smaller, the barrier/wetting layer becomes a larger portion of the interconnect structure. In order to maximize the benefit of copper""s low resistivity, the diffusion barrier/adhesion layer must be made very thin and/or must have low resistivity itself (so that it does not impact the effective line resistance of the resulting metal interconnect structure). As is readily apparent, depending on the device to be fabricated, various methods have been used in an attempt to develop a tantalum film which is a phase when lower resistivity is desired. Typically, small additions of nitrogen have been made to tantalum films to lower the resistivity of the tantalum. This method is difficult to control, as any deviation in the nitrogen content (evenxc2x11 sccm of nitrogen flow) may lead to a significant increase in resistivity. Another proposed sputter deposition method for lowering resistivity involves control of the ion energy striking the substrate (via grounding of the substrate). However, this method does not always produce reproducible results, is sensitive to substrate cleaning and preparation, and affects the film stress. Care must be taken to avoid high film stress so that the barrier/wetting layer does not tend to separate or pop off the substrate upon which it is deposited.
After deposition of the tantalum-comprising barrier/wetting layer, any of the tantalum-comprising material deposited on the substrate in areas other than the conductive interconnect structures must be removed. Whether the tantalum-comprising material is removed by ion bombardment techniques (e.g., reactive ion etching) or by CMP, the difference in hardness between the tantalum-comprising material and copper causes problems. Residual material from the copper deposition is rapidly removed, leaving residual material from the tantalum-comprising barrier layer. Continued ion bombardment or CMP to remove the tantalum-comprising material can result in the undesired removal of adjacent copper which is intended to make up the interconnect structure.
It would be highly desirable to have an ultra-low resistivity tantalum film which exhibits low stress (tends to stay bonded to underlying layers), and which is more easily polished using CMP, so that its polishing rate is closer to that of copper.
We have discovered that, by depositing a tantalum layer at particular substrate temperatures, it is possible to obtain a lower resistivity than has previously been known in the literature. The low resistivity material has been obtained by sputter deposition at an elevated substrate temperature ranging from about 325xc2x0 C. to about 550xc2x0 C., or by sputter deposition at a substrate temperature of less than about 325xc2x0 C. followed by annealing at a temperature of about 325xc2x0 C. or greater. High density plasma sputtering techniques which provide for ion bombardment of the film surface can be used in combination with the elevated substrate temperature, to add momentum energy to the film surface, thereby reducing the substrate temperature required, while still providing the ultra-low resistivity tantalum film. A reduction of about 40% in the required substrate temperature can be obtained by this means, while maintaining a reasonable film stress and a reasonable film deposition time period.
In a less preferred, alternative method, a small amount (less than about 15 atomic %) of nitrogen may be added to the tantalum film (while depositing the film at an elevated temperature) to obtain the lower resistivity at a slightly lower substrate temperature, in the range of about 275xc2x0 C. This provides a lower resistivity than previously known for nitrogen addition, while reducing the process temperature necessary to obtain the lower resistivity.
In another development, we have discovered that the tantalum layer produced by the method of the present invention can be more readily removed by the CMP polishing techniques used for removal of excess, residual metal in the damascene process. The low resistivity tantalum is more flexible, more easily cleaved, and more easily polishedxe2x80x94an advantage in the damascene process when it is desired to remove excess metal from the field surface of a device structure. This is particularly helpful when copper is used as the interconnect metal, since copper polishes easily and it previously took 4 times longer to remove tantalum barrier layer then to remove excess copper from a field surface. The low resistivity tantalum deposited using the present method exhibits an increase in CMP polishing rate of nearly 50% over the previously known CMP polishing rate for tantalum films.